1. Field of the Invention
This disclosure is directed to a method of preparing a low resistance metal line, a patterned metal line structure, and a display device using the same. More specifically, the method is directed to the preparation of a low resistance metal line, in which a wet plating process is used instead of a vacuum film forming process, thereby simplifying the process and decreasing the cost. In addition, the method is also directed to the formation of a self-assembled monolayer (SAM) in which the adsorption density, and the strength of the metal catalyst are increased, resulting in the formation of a high-density metal catalyst layer, and thereby obtaining a high-quality metal line.
2. Description of the Related Art
As electronic apparatuses become miniaturized and more highly integrated, the line width subsequently becomes narrow, resulting in an increase in the metal line resistance and the signal delay, undesirably causing problems related to decreased display quality. In particular, these problems impede the development of thin film transistor-liquid crystal displays (TFT-LCDs) that simultaneously possess high image quality and a large surface area.
Typically, liquid crystal displays (LCDs), for example, a flat panel display device, adopt a driving method for applying voltage to a display material, using for example, liquid crystals interposed between a pair of substrates. In this example, an electrical line formed of conductive material is arranged on at least one substrate.
For example, an active matrix driving type LCD, comprises a pair of substrates having a display material interposed therebetween. On any one of the active matrix substrates, a gate electrode and a data electrode are disposed in a matrix arrangement, and in the area where the gate electrode and the data electrode intersect, a thin film transistor (TFT) and a pixel electrode are disposed. The gate electrode and the data electrode are primarily formed of a metal, such as, for example, tantalum (Ta), aluminum (Al), or molybdenum (Mo), and are provided in the form of a film produced using a dry film forming process, such as sputtering.
In the case where it is desired to produce a flat panel display device having an increased area with improved resolution, the drive signal delay is regarded as a significant problem. The problems are attributable to the increase in the line resistance and the increase in parasitic capacitance in proportion to the increase in drive frequency.
Therefore, in order to solve the problem of drive signal delay, attempts have been made to use copper (Cu) as the line material in place of conventionally used Al, α-Ta, and Mo, due to the lower electrical resistance and superior charge mobility of Cu, Because Cu is a material having relatively low resistivity and superior electro-migration resistance, significant effort has been undertaken to develop various new techniques that make use of these properties of copper. For example, in the chapter “Low Resistance Copper Address Line For TFT-LCD” (Japan Display 1989, p. 498-501), the use of Cu as a material for a gate electrode in a TFT-LCD are disclosed.
Further, the above reference discloses the need for the improvement of adhesion so that a metal film, such as Ta, may be utilized as a substrate, because the Cu film, which is formed through sputtering, has poor adhesion to a glass substrate.
In the case where both the Cu film for decreasing the resistance and the substrate-metal Ta film for improving the adhesion of the Cu film are formed using a vacuum film forming technique, such as sputtering, separate film forming processes for the Cu film and the substrate-metal Ta film are utilized. Furthermore, there are also separate etching processes for the Cu film and the substrate-metal Ta film. Consequently, the number of processes is increased, thereby undesirably increasing the cost of preparation. In addition, as the area of the display is increased, that is, as the film forming area is enlarged, large-scale vacuum film forming and etching devices are needed, resulting in a further increase in production costs.
Accordingly, the establishment of a wet plating technique for the preparation of a Cu line that can be used to form a film at a low cost, and without the use of the vacuum film forming device, is desired.
Electroless plating is among the methods used for the formation of a copper line. Electroless plating is a method that takes advantage of the difference in ionization tendency between a reducing agent and an oxidizing agent present in a plating solution. The method comprises activating the surface of a substrate and then plating a desired film on the substrate. Because the electroless plating method is applied uniformly over the entire surface of the substrate, without the use of an external power source, it effectively realizes a low production cost and a simple process, resulting in outstanding productivity.
In the electroless plating method, a metal film is directly plated on a diffusion barrier using an electrochemical process. Consequently, the fine interfacial structure between the diffusion barrier and the metal film, and the interfacial reaction, considerably affect the properties of the metal line, including the electrical properties and thermal stability. Further, in the electroless plating method, a metal catalyst nucleus is formed through activation prior to the formation of the metal plating layer. The metal catalyst nucleus functions as a catalyst upon plating, thereby facilitating the plating process.
However, the electroless plating method is unsuitable for practical applications because the plated film, obtained through activation using tin and palladium, has poor adhesion.